The present invention relates to a method of draining and venting the permeate gases that diffuse into the annulus of a flexible tubular pipe and to a pipe suitable for implementing such a method.
The flexible tubular pipes involved here are unbonded pipes such as those described in the API (American Petroleum Institute) standard 17J to which the reader may refer.
These flexible tubular pipes used in the field of offshore oil production are intended for transporting fluids, especially hydrocarbons. They comprise at least one flexible internal tube made of a polymer material, more commonly called an internal pressure sheath, within which said hydrocarbons flow. The pipes include an external sheath and reinforcing plies or armors, located in the annular region, between the external sheath and the internal pressure sheath.
The pipes, depending on their application and especially on their service conditions and the fluid transported, may include a carcass placed inside the pressure sheath.
In addition, they are likely to include what is called an intermediate sheath located in the annular region.
Although the internal pressure sheath is impermeable to the hydrocarbons, certain gases and molecules contained in these hydrocarbons are liable to diffuse through the wall of said pressure sheath depending on the material of which it is made, on the concentration of said gases and molecules in the hydrocarbons and on the local conditions, especially the pressure and temperature conditions. These gases, which diffuse through the wall of the internal pressure sheath and will be called permeate gases, comprise in particular water in the vapor phase, carbon dioxide, methane or hydrogen sulfide.
Said annular region, comprising the armor(s) or reinforcing plies, includes flow paths that extend, for example, around the armor elements and along said internal pressure sheath over the entire length of the pipe, said permeate gases being capable of flowing naturally along said flow paths, to be drained away toward a venting device. Most often the venting device is located in the connection end-fittings and is formed from one or more differential valves. The flow paths are for example formed by the gaps that exist between the armor wires of the reinforcing plies, or armor plies, or else, thanks to the lateral faces of the ply wires that are longitudinally profiled or grooved and that, even in content, are suitable for forming said flow paths.
Two problems then arise. Not only are the permeate gases liable to corrode the elements of the reinforcement, which are generally made of steel, but water in the vapor phase is also likely to condense under certain temperature and pressure conditions, forming a liquid mixture which obstructs the flow paths and may block the natural flow of the permeate gases. As a result, the pressure of the permeate gases increases in the annular region and these gases, in particular carbon dioxide and hydrogen sulfide, stagnate between the elements of the reinforcement, thereby further exacerbating the corrosion.
Furthermore, the build-up of permeate gas and condensate in the annular region is also likely to cause the outer coating that protects the reinforcement and the internal pressure sheath to burst when the pressure in the annular region is above the pressure existing on the outside of the pipe. This risk is lower at great depth since the hydrostatic pressure compensates for the pressure in the annular region. In contrast, this risk is greatest near the surface, when the permeate gases are no longer being vented.
This condensation problem may in particular be critical in what are called S or wave (lazy-S, steep-S) riser configurations. This is because the condensation in the top part of the pipe will concentrate at the low point of inflection of the pipe (the sag bend) preventing the natural drainage in the bottom part of the pipe from taking place along the pipe toward the surface end-fitting.
To prevent the permeate gases from building up in said annular region, it has been envisioned to drill holes in the outer coating and to close off these holes by valves that open and close rapidly, theses valves being designed to open and close at defined differential .pressures between the annular region and the outside. Thus, when the pressure of the permeate gas in the annular region is above said defined differential pressure, the valves open in order to release the permeate gas to the outside, and close rapidly so that nothing penetrates into the annular region. This is because it is vital for no fluid to penetrate into the annular region, in particular seawater, when the flexible tubular pipe connects a subsea wellhead to a surface platform.
The reader may in particular refer to document EP 0 341 144 which describes such a permeate gas venting device placed in a connection end-fitting of a pipe.
However, on the one hand there still remains a risk that the valves remain in the open position, although the pressure in the annular region is below the external pressure, in which case water can penetrate into the annular region, and, on the other hand, at great depth, the external pressure is such that the valves cannot be easily opened and that the permeate gases then condense in the annular region.
Thus, it has been envisioned to let the permeate gases condense and to reinject the condensate into the internal pressure sheath where the hydrocarbon is flowing. Such a method is described in document WO 00/17479.
However, such a method requires relatively powerful and possibly submersible pumping means.
One problem that arises, which the present invention aims to solve, is therefore to preserve the flexible tubular pipe while not only preventing the permeate gases from being able to condense in the annular region but also preventing the external water from being able to penetrate thereinto.